This invention relates to nanometer-period optical gratings and superlattices from which such optical gratings can be fabricated.
Superlattices are epitaxially-grown material structures having atomic-scale periodic compositional variations. The more familiar superlattices are fabricated from one or more semiconductors which are grown in alternate layers, one or more atoms thick, on a major surface of a suitable substrate. A typical superlattice is exemplified by alternate layers of AlGaAs and GaAs grown on a major crystallographic plane of GaAs. Such conventional superlattices may be termed "vertical" superlattices in that the periodicity in composition is in the direction perpendicular to the substrate surface.
More recently, "horizontal" superlattices in which the compositional periodicity is in a direction substantially parallel to the substrate surface have been developed. Horizontal superlattices together with the method of fabrication is fully described in U.S. Pat. No. 4,591,889 issued on May 27, 1986 which is incorporated by reference.
The availability of horizontal superlattices provides the basis for fabricating ridged surfaces with periods an order of magnitude smaller than has heretofore been possible. Such gratings will be termed "ultragratings" in the material that follows.